The field of the invention is the production of alkyl esters of saturated aliphatic carboxylic acids and the present invention is particularly concerned with reacting olefins with carbon monoxide and alkanol in the presence of a catalyst consisting of a colbalt compound and a promotor selected from pyridine, non-ortho-substituted alkylpyridine or mixtures thereof at elevated pressures and elevated temperatures.
The state of the art of such alkoxycarbonylation reactions may be ascertained by reference to U.S. Pat. Nos. 3,507,891; 3,906,016 and 4,041,057 and the article "Hydrocarboxymethylation--an Attractive Route from Olefins to Fatty Acid Esters?" by Peter Hofmann et al as published in I & EC, Product Research & Development, Vol. 19, Sept. 1980, pp. 330-334, the disclosures of which are incorporated herein.
It is known that by reacting olefins with carbon monoxide and a compound having a replaceable hydrogen atom such as an alkanol in the presence of a catalyst containing a metal of Group VIII of the Periodic Table of elements and possibly a promotor, fatty acid esters can be produced as disclosed in J. Falbe, Synthesen mit Kohlenmonoxid, Springer publishers, Berlin, Heidelberg, New York (1967).
An especially preferred variation of this reaction, which is termed alkoxycarbonylation, is the conversion in the presence of cobalt catalysts. The rate, the selectivity and the yield in linear fatty acid esters of the cobalt reaction can be increased by adding promoters belonging to the pyridine class of compounds. Pyridine itself and also non-ortho-substituted alkylpyridines and mixtures thereof have been found particularly effective.
The application of such promoters suffers from a substantial drawback in that the cost of pyridine and non-ortho-substituted alkylpyridines is relatively high and moreover that these compounds are not always available in the required commercial quantities.
In addition to the costs of the pyridine or the non-ortho-substituted alkylpyridine used as promoter, the economy of a alkoxycarbonylation process is also affected by, among other factors, the achievable ester selectivity and by the usefulness of the by-products.
Whereas the utilization of the paraffin formed as by-product by the olefin hydrogenation, or of the aldehyde formed by hydroformylation as a rule causes no difficulty--paraffins for instance can be used again to prepare the initial olefin, aldehydes can be used just as esters for the production of alcohol or of carboxylic acid--to date no economically significant utilization has been found for the mixture of substances obtained as byproduct, which is of a higher boiling point than the esters formed as reaction product and the pyridine, non-ortho-substituted alkylpyridine or mixtures (high boiling point substances) used as the catalytic component.
In order to prevent an enrichment of these generally high boiling point substances, which in reprocessing by distillation of the alkoxycarbonylation mixture collect together with the catalyst as distillation sump substances, when the reaction is repetitive or continuous, they are either separated by expensive reprocessing methods from the catalytic metal and discarded as disclosed in European published application No. 0 008 024 and German Pat. No. 921,988, or are burned off together with the catalyst as disclosed in British Pat. No. 2,005,652 and U.S. Pat. No. 4,041, 057.